1. Field of the Invention
The present invention relates to methods for constructing a foundation for an offshore wind power generation system for generating electrical power from offshore wind energy, where the foundation holds the wind power generation system above the sea surface, and supports a tower that extends down to a level close to the seabed. More particularly, the present invention relates to methods for constructing a foundation for a wind power generation system having piles or a caisson as a base.
2. Description of the Background Art
Offshore wind power generation systems typically generate electrical power by generators by using rotation of wind turbines by offshore winds. Such a wind power generation system is supported on the upper end of a tower that extends from a foundation provided on the seabed, to a position in the air through the sea surface. That is, the wind power generation system is supported by the foundation provided on the seabed. Such a foundation for supporting a wind power generation system and a tower is disclosed in, e.g., Japanese Patent Publication No. 2005-194792 of unexamined applications. According to Japanese Patent Publication No. 2005-194792 of unexamined applications, a wind power generation system and a tower are supported by a foundation, which includes a plurality of steel pipe piles, a base receiving frame made of a steel plate, and a base block made of reinforced concrete. The plurality of steel pipe piles are respectively positioned at the vertexes of a regular polygon as viewed in plan, and are driven into the seabed. The base receiving frame is fixed on the plurality of steel pipe piles, and has holes at the positions corresponding to the steel pipe piles, for respectively inserting heads of the steel pipe piles therethrough. The base block is fixed on the base receiving frame, and has holes for respectively inserting therethrough the heads of the steel pipe piles protruding from the base receiving frame. A base end of the tower is fixed on the base block so that the center of the tower is positioned at the center of the regular polygon.
As described above, the wind power generation system and the tower are supported by the foundation. However, foundations having an insufficient bearing force may cause, for example, damage to the wind power generation system due to the failure to suppress vibration generated in the wind power generation system, and collapse of the tower due to propagation of the generated vibration to the tower.
Conventional foundations for supporting a wind power generation system and a tower have the following problems.
Since the steel pipe piles driven into the seabed serve as resistance to the tidal currents, an excess flow is generated around the steel pipe piles near the seabed. Such an excess flow causes a phenomenon called “scouring,” a phenomenon that the seabed is worn away and chipped off, near the steel pipe piles driven into the seabed. This gradually scoops out accumulated earth and sand and exposes the steel pipe poles, whereby the bearing force as a foundation can be reduced.
In an undersea part of the tower located below the sea surface, the steel pipe piles under the sea are subjected to tidal waves, whereby a moment is generated near the seabed in the foundation. This can produce a gap between each steel pipe pile and the surrounding ground, whereby the bearing force as a foundation can be reduced.
More over, the wind power generation system and an aerial part of the tower located above the sea surface are subjected to offshore winds, whereby a moment is generated near the seabed in the foundation and this can reduce the bearing force as a foundation, as in the above phenomenon.